A disease of African marigolds, Tagetes erecta, was recognized in the 1930's in which the leaves of the plants were curled and abnormal and also densely covered by necrotic spots. In 1955, Danish plant pathologist Ernst Hellmers demonstrated that the disease was caused by a Pseudomonas bacterium, Acta Agric. Scand. 5:185-200 (1955). In investigating an outbreak of disease causing losses of marigold seedlings of Tagetes erecta in Australia characterized principally by apical chlorosis, cultures of Pseudomonas bacteria were again isolated. It was found in this case that cell-free culture filtrates of cultures of the pathogenic bacteria were also phytotoxic, indicating that the agency of pathogenesis of the bacteria was an exotoxin capable of blocking chloroplast development in growing plant tissue but which does not otherwise appear to effect existing chlorophyll in plants. Aust. J. Agric. Res. 29:831-839 (1978). The toxin produced by the bacteria, which is now referred to as Pseudomonas syringae pv. tagetis, has been referred to as tagetitoxin. Tagetitoxin has been demonstrated to be a low molecular weight heterocyclic organic compound. The structural data available for the compound suggests that it includes an eight membered ring, with a sulfur atom in the ring, and also includes an amino group and a phosphate group joined to points on the ring. Phytochemistry 22:1425-1428 (1983). The proposed structure of tagetitoxin is as follows: ##STR1##